The present invention generally relates to an electrical filter, and more particularly, to a parallel connection multi-stage band-pass filter suitable for use as a channel filter of a radio frequency signal combining/sorting device for a mobile unit communication system such as an automobile telephone or the like, or as a transmission/receiving filter for mobile equipment, etc.
A recent trend in mobile unit communication systems such as automobile telephones and the like, has been to employ a so-called cellular system. Because of the rapid increase in the number of users, a reduction in the cell radius and an increase in the number of base stations employed has been concurrently required. In accordance with this trend, features such as compact size, low loss, and cost reduction are required in the radio frequency signal combining/sorting device employed in base stations.
As shown in FIG. 16, a conventional radio frequency signal combining/sorting device to be used for the base station of the cellular system, includes a plurality of sets of isolators 1 and channel filters 2, a power composition network 3 for interconnecting these sets of isolators 1 and channel filters 2, an antenna monitor 4, and one antenna filter 5, coupled to each other as shown.
In a conventional radio frequency signal combining/sorting device of the above described type, each channel filter 2 is constituted by a band-pass filters (abbreviated as BPF) which allows signals of a specific frequency band corresponding to a respective channel to pass therethrough.
The conventional band-pass filter of the above described type is designed as described hereinbelow to realize a filter for actual applications.
Referring to FIG. 17, the circuit for the band-pass filter as described above is obtained by subjecting a low-pass filter (abbreviated as LPF) for which a generally known designing theory exists, to circuit conversion by a conversion formula called an inverter. The band-pass filter circuit thus obtained by such circuit conversion consists of a series connection multi-stage band-pass filter circuit 8 in which a plurality of neighboring LC resonance circuits 7 are sequentially subjected to mutual inductive coupling as shown in FIG. 17.
The series connection multi-stage band-pass filter circuit 8 of FIG. 17 as referred to above is the so-called design circuit for designing the band-pass filter, and is characterized in that it is readily realized in a microwave band region.
For realizing the series connection multi-stage band-pass filter circuit 8 which is the design circuit obtained in the above described manner as an actual filter, the respective LC resonance circuits 7 in three stages, connected in series to each other as in FIG. 17, are replaced or approximated by actual resonators, for example TE.sub.01.delta. dielectric resonators. Accordingly, a series connection multi-stage band-pass filter including a plurality of dielectric resonators and having predetermined frequency characteristics can be realized.
One example of the series connection multi-stage band-pass filter 11 thus realized is illustrated in FIG. 18, with its equivalent circuit shown in FIG. 19.
The series connection multi-stage band-pass filter 11 referred to above is the same filter proposed by the present inventors in "Dielectric high-power band-pass filter using quarter-cut TE.sub.01.delta. image resonator for cellular stations," IEEE transactions on Microwave Theory and Techniques, MTT, Vol. 35, No. 12, pp. 1150-55, December 1987. As shown in FIG. 18, each series connection multi-stage BPF 11 includes a plurality of arcuate dielectric resonators 12 referred to by the present inventors as "quartercut TE.sub.01.delta. image resonators" having been formed from a 1/4 portion of the TE.sub.01.delta. mode dielectric resonator which was originally in an annular shape. Ceramic substrates 14 formed with electrode films on surfaces 13 thereof are disposed in an L-shape to act as an electric wall. The arcuate dielectric resonators 12 are fixed to the ceramic substrates 14 at predetermined intervals. The ceramic substrates 14 and dielectric resonators 12 function as TE.sub.01.delta. mode image resonators. The ceramic substrates 14 referred to above are electrically and mechanically fixed onto walls of a metallic housing 15, whereby the interior of housing 15 is formed to have the construction equivalent of a TE.sub.01.delta. mode circular out-off type waveguide divided into 1/4. Each of the dielectric resonators 12 in FIG. 18 is inductively coupled to another, with the dielectric resonators 12 at the ends of the series circuit inductively coupled with external loads.
The series connection multi-stage band-pass filter 11 as described above may be made much smaller in size as compared with a filter employing an ordinary cavity resonator because series-coupled resonators of this kind are generally arranged such that respective natural vibration modes take care of the respective frequency components. However, with dielectric resonators 12 connected in series, energy distribution will differ among the dielectric resonators 12 at respective stages.
FIG. 7 shows one example of band-pass characteristic and group delay characteristic obtained by a conventional three stage series connection multi-stage band-pass filter.
As is seen from the group delay characteristic in FIG. 7, it is difficult to realize a flat group delay characteristic over the entire band-pass region using a series connection multi-stage BPF, because respective frequency components share the same respective natural vibration modes, and external coupling degrees for all natural vibration modes are correlatively altered even when parameters for the resonators are adjusted, and thus, a flat group delay characteristic at each respective resonance frequency cannot be set as desired.
Moreover, the group delay characteristic of the series connection multi-stage band-pass filter is characterized in that it has peak values at the extreme ends of the band-pass region. In order to obtain a flat group delay characteristic over the entire working band region, the design region must be broadened so that the peaks at the extreme ends are located outside the working band region, thereby making it difficult to realize band-pass characteristics with sufficiently superior selectivity.
Furthermore, the group delay characteristic as described above cannot fully handle the digitized transmission signals employed in the rapidly advancing technology of today. Therefore, the realization of filters having a flat group delay characteristic is in great demand.
Incidentally, the filter 11 realized in the manner described above is only an approximation of a band-pass filter, derived from the design theory of a low-pass filter, employing a TE.sub.01.delta. mode dielectric resonator 12, and it does not have the electrical characteristics which fully agree with the design characteristics possessed by the design circuit referred to earlier.
Therefore, Zuiho KYO et al. have proposed a parallel-coupled circuit simulation model utilizing an inherent mode developing method for simulating the actual series-coupled filter described above, on pages 9 to 16 of a paper entitled "Composition of microwave circuit by inherent mode developing method," Electronic Communication Institute, Microwave Research Meeting Data, MW82-54, 1982. The parallel-coupled circuit simulation model has a circuit construction as shown in FIG. 20 and is intended to allow simulations incorporating factors such as the asymmetrical nature of attenuation and the spurious modes in a microwave filter. In a case where a series connection multi-stage band-pass filter has natural vibration modes of m pieces, this model assumes that when the respective variation modes are realized, for example, by continuous resonators comprised of n=3 pieces, the natural vibration modes of m pieces are obtained by multi-stage resonators of (m.times.n) pieces. It should be noted that when the relation is n=3, for example, the mutual coupling of the three resonators connected by series coupling reduces the number of degrees of freedom of the respective natural vibration modes to 7, whereas the degrees of freedom of the electrical characteristics derived from the assumption that each resonator is independent and parallely coupled, as is done in the above model, would be 9.
Although the above simulation model is extremely useful for theoretical analysis of the natural vibration modes of a series connection multi-stage band-pass filter, this model cannot be used as described above for actual filters.